Nitrogen monoxide (NO) has been implicated in a number of diverse physiological processes, including smooth muscle relaxation, platelet inhibition, nerve transmission, immune regulation and penile erection. Nitric oxide is produced under various conditions by virtually all nucleated mammalian cells. A number of pathologies are ascribed to abnormalities in NO production including stroke, insulin dependent diabetes, septic shock-induced hypotension, rheumatoid arthritis and multiple sclerosis. Nitric oxide is synthesized in biological tissues by an enzyme called nitric oxide synthase (NOS) which uses NADPH and molecular oxygen to oxidize L-arginine to citrulline and nitric oxide.
Nitric oxide synthase (NOS) exists in at least three isoforms, which fall into two primary categories: constitutive and inducible. Two constitutive isoforms, which are calcium and calmodulin dependent, have been identified, and one inducible isoform has been identified. The constitutive isoforms are (1) a neuronal isoform, NOS-1 or nNOS, which is found in the brain and skeletal muscles and (2) an endothelial isoform, NOS-3 or eNOS, which is expressed in the endothelium of blood vessels, the epithelium of the bronchial tree and in the brain. These constitutive isoforms are not the target of the NOS inhibitors of the present invention.
The inducible isoform (NOS2 or iNOS) is expressed in virtually all nucleated mammalian cells following exposure to inflammatory cytokines or lipopolysaccharide. Its presence in macrophages and lung epithelial cells is particularly noteworthy. The inducible isoform is neither stimulated by calcium nor blocked by calmodulin antagonists. It contains several tightly bound co-factors, including FMN, FAD and tetrahydrobiopterin.
Nitric oxide generated by the inducible form of NOS has been implicated in the pathogenesis of inflammatory diseases. In experimental animals, hypotension induced by lipopolysaccharide or tumor necrosis factor a can be reversed by NOS inhibitors. Conditions which lead to cytokine-induced hypotension include septic shock, hemodialysis and interleukin therapy in cancer patients. It is expected that an iNOS inhibitor would be effective in treating cytokine-induced hypotension. In addition, recent studies have suggested a role for NO in the pathogenesis of inflammation, and NOS inhibitors would therefore have beneficial effects on inflammatory bowel disease, cerebral ischemia and arthritis. Inhibitors of NOS may also be useful in treating adult respiratory distress syndrome (ARDS) and myocarditis, and they may be useful as adjuvants to short term immunosuppression in transplant therapy.
The diversity and ubiquity of NO function in physiology make the specific therapeutic targeting of NO-related phenomena an important consideration. Since endogenous NO production is the result of the actions of related but distinct isozymes, the differential inhibition of NOS isozymes allows more selective therapy with fewer side effects.